1. Field of the Invention
This invention relates to the preparation of a natural fertilising material. More particularly, this invention relates to a method and a plant for preparing an essentially sterile fertilising material from waste sewage sludge.
It is well known that before waste sewage sludge can be used for agricultural or horticultural purposes, essentially complete sterilisation of such sludge is required because of the particular contents of the sludge, which may contain parasites, viruses, bacteria, and the like. Without some or other sterilisation process being applied to the sludge, the danger of propagation of and/or subsequent contamination and infection by parasites and disease precludes the free use of such sludge for agricultural or horticultural purposes.
Since human faeces carry virtually all human and animal infections and parasites, it is a prerequisite for the substantially unrestricted use of such a natural fertilising material in agricultural or horticultural applications that the sludge is suitably treated by a sterilisation process in order to produce a safe fertilising material.
2. Description of Related Art
It is known to those skilled in the art that waste sewage sludge is often matured after treatment thereof in a sewage plant for sterilisation purposes. Such maturation is carried out either by leaving a pile of waste sewage sludge exposed, in which case fungus may form on the surface thereof; or a pile of sewage sludge may be placed under a layer of, for example, grass sods, in which case natural processes lead to the generation of heat in the pile of sludge. Both these maturation processes usually take several weeks before the sludge may be recognised as being suitable for use as a natural fertilising material. Whilst these natural processes take a considerable period, which is a disadvantage in terms of throughput or production of such natural fertilising material, these processes do not invariably rid the sludge of undesiragble parasites, viruses, bacteria, and the like, and give rise to the loss of nitrogen due to volatilisation, which results in a lower nutrient value in the product.
In contrast to the above natural methods, it is also known to those skilled in the art that sewage effluent may be treated by means of exposure to chlorine gas, the sludge in this case being in suspension, i.e. at a very high moisture content. Such methods, and for example filtration followed by break-point chlorination, can clearly not be applied to sludge. It has been reported that even such treatment cannot guarantee inactivation of all the organisms present.
U.S. Pat. No. 3,445,383, dated May 20, 1969, to Roland J. Horvath et al. teaches a process for treating sewage effluent with chlorinated glycolurils in order to achieve slow release of chlorine to provide long term disinfection of sewage effluent. Essentially, however, this amounts to treatment of sewage effluent with chlorine, and this method, as pointed out above, cannot guarantee a sterile product.
In 1975, Smith, Young and Dean (Water Research, Vol. 9, pp. 17-24) proposed a process of aerobic thermophyllic digestion, or so-called "wet composting", to sterilise sewage sludge, but the method does not seem to have been widely adopted, probably because of its high cost.
The presently accepted methods for ensuring the destruction of pathogens present in sludge usually involve the application of heat. It is generally accepted that Ascaris ova, considered to be the most resistant of the organisms present in sewage and therefore an indicator of the efficacy of a treatment method, are destroyed when subjected to a temperature of at least 60.degree. for a minimum period of 30 minutes (B.D. Hays, Water Research, Vol. 11, pp. 583-95, 1977). In Germany and Switzerland, sludge intended for land application is generally treated in this manner; use of such heat sterilisation is, however, considered too expensive a procedure for most other countries. As a result, large quantities of sewage sludge infected by pathogens have been accumulating in many countries, particularly on the fringes of urban areas, where they constitute a continuous health hazard, without their soil nutritional values being utilised.
Another recently developed technique which is currently applied for the sterilisation of sewage effluent and of sludge is the process of radiation, which involves particularly the use of gamma rays or of electrons of relatively low energy (i.e. velocity). Plants using gamma ray sterilisation are generally very expensive and have a modest throughput.
Methyl bromide has been used routinely over many years for fumigating soils prior to the planting of crops sensitive to insect pests and organisms such as nematodes, or in the preparation of seedbeds. In such applications the methyl bromide is injected into the soil under an impervious cover (to prevent escape of the methyl bromide into the atmosphere) at ambient temperature in the form of the liquid and/or as the vapor.
In the United States Department of Agriculture Publication No. E-838 entitled "Methyl bromide fumigation of cotton seed in freight cars for the destruction of pink bollworms" by G.L. Philips the use of methyl bromide vapor is discussed for killing pink bollworm larvae embedded in cotton seed when exposed to moderate concentrations of methyl bromide. In U.S. Pat. No. 4,200,656, Cohen et al describe a method for fumigating grain stored in bins by a gravity penetration method comprising applying a mixture of carbon dioxide and methyl bromide. All insects were killed throughout the total depth of the bin. In a control experiment using methyl bromide alone, only insects in the upper part of the bin were killed.
The fumigation techniques taught by these references, however, fall short of sterilisation, i.e. they do not mention Ascaris ova nor do they suggest that they could be effective against Ascaris ova.
In the Textbook of Parasitology by David L. Belding (third edition), on pages 468 and 469, mention is made that Ascaris and Toxocara ova in infested soils are killed by the use of free iodine, and green or root vegetables from gardens fertilised by night soil can be safely used after treatment with free iodine. It is also stated that treatment of infested soil by means of agricultural and other chemicals, for example chlorthion and isochlorthion, is claimed to kill the ova.
Belding (on p. 469 of his book) quotes K. Enigk and J. Eckers (Zentrallbl. Bakt. Abt. 1 Orig. Volume 179, pp. 397 -432, 1960) as having found that "gaseous dibromethane" "rapidly" kills Ascaris ova in soil. Of course, there is no compound known in English by this name. Reference to the original paper by Enigk and Eckers shows that, while the English Summary accompanying the paper uses the term "dibromethan", the compound should correctly have been translated from the German text as dibromoethane. Belding therefore used a wrong term, which we repeated unchanged in the specification of our prior copending U.S. application Ser. No. 315,567, now abandoned, and which we interpreted at the time as meaning dibromomethane.
In their experiments Enigk and Eckers sprayed the liquid dibromoethane (of boiling point 131,4.degree. C.) by means of a garden insecticide spray on to the surface of experimental patches of soil which were artificially infected with Ascaris ova. At soil temperatures of 20.degree.-21.degree. C. the application of 100-200 ml/m.sup.2 of surface area of this compound produced essentially complete eradication of Ascaris ova up to a depth of 20 cm, but at lower soil temperatures and/or application rates the efficacy of the treatment was considerably reduced.
Although Belding in his book makes no reference to it, we have now noted that the Enigk & Eckers paper also reports experiments (aimed at finding an effective disinfectant to destroy Ascaris ova in soil) with pressurised liquid methyl bromide, applied to the top of the experimental patch of soil through a small number of pipe nipples located in an impervious cover placed over the soil. The authors report that methyl bromide was tested on surfaces of 25 to 45 m.sup.2 in quantities of 22,2 to 44,4 ml/m.sup.2, but showed no reliable effect in that the Ascaris ova were killed only in the immediate vicinity of the nipples, whereas in positions some distance away from the nipples the effect was small or the treatment failed completely.
Thus these references teach that the use of organic bromo compounds such as dibromoethane and methyl bromide as a means of killing Ascaris ova in soils, will meet with limited success. Efficient eradication was observed only up to a few centimeters deep from the surface of the soil, and in some cases only in the vicinity of the point of application of the chemical. In fact, Enigk and Eckers teach away from the invention, in that they specifically state dibromoethane to be more successful in eradicating Ascaris ova than methyl bromide, and in that they teach that methyl bromide "showed no reliable effect" (cf their English Summary on p. 427 of the paper). We now suspect that the failure of their experiments to produce more reliable results with methyl bromide is almost certainly due to the fact that they applied the methyl bromide, at least partially, in the form of the liquid directly from the pressurised container, as would appear to be suggested by the photograph appearing on p. 405 of their paper, which has the caption : "Abb. 1 Desinfektion des Erdbodens mit Methylbromide. Das Methylbromid stroemt aus einer Stahlflasche durch einen Einfullstutzen unter die Kunststoffplane", which may be translated as: "FIG. 1. Disinfection of the ground with methyl bromide. The methyl bromide flows out of a steel bottle through a filling nipple under the plastics cover. " The fact that they express the amounts of methyl bromide applied per m.sup.2 of ground in millilitres further corroborates this interpretation. It is suggested by the applicant, with the benefit of hindsight derived from his own invention, that the methyl bromide, when applied in this manner, would fully permeate the surface of the ground before it could spread laterally to achieve a uniform distribution in the horizontal plane. Dibromoethane, because of its high boiling point, can only practically be applied by spraying the liquid; the high boiling point also makes it safe to do so without a cover (in contrast to methyl bromide), making it possible to observe the application of the chemical and ensure that it is uniformly applied.
The results of Enigk and Eckers are, in any case, not applicable to the treatment of sewage sludge, as it is not obvious whether the organic bromo compound vapor would produce a similar degree of penetration in the sewage sludge compared to soil, having regard to the differences in particle size, density, compaction and moisture content of the two materials.
Despite the great need for an efficient treatment method of reasonable cost that will eradicate Ascaris ova in sewage sludge, thereby to effectively sterilise the sludge, and thus to make it available as a natural fertiliser material, the use of methyl bromide has not been reported.